The Laboratory
Maximum Load Lab
Teacher’s Guide
Topic:
Vectors and Forces
The following information is provided to the student:
Question:
What is the maximum load which a light string can support when assuming a v-shaped orientation
and supported on its ends at two points? How does the measured value compare to the predicted
value?
Purpose:
To determine the maximum load which a light string could support when assuming a v-shaped
orientation and supported on its ends at two fixed points and to compare the maximum value to the
predicted value based on the string's breaking strength.
A complete lab write-up includes a Title, a Purpose, a Data section, and a Conclusion/Discussion of
Results. Data section should include a diagram of the experimental setup and a representation of how
the measurements of maximum load was determined. Results of several trials are documented and an
average value is determined; outliers are indicated. Calculations of the theoretically predicted value is
included; analysis is thoroughly documented beginning with a force triangle. The
Conclusion/Discussion answers the question posed in the Purpose. An error analysis along with a
percent difference calculation is included in the discussion.
Materials Required:
Spool of light string (or thread); protractor; spring scale or computer-interfaced force probe; two hooks
from which to suspend the string.
Description of Procedure:
Students are given the same string used in the Breaking Strength Lab in the Newton's Laws unit. The
string is hung from two vertical supports, so as to create a v-shaped orientation (roughly). Students
devise a means of exerting a force at the mid-point of the string in order to determine the maximum
force which the string can withstand before breaking.
Alternative Materials and Procedure:
Lab poles and clamps could be used to provide supports from which to suspend the string.
Safety Concern:
There is always a higher than usual level of risk associated with working in a science lab. Teachers
should be aware of this and take the necessary precautions to insure that the working environment is as
safe as possible. Student horseplay and off-task behaviors should not be tolerated.
Suggestions, Precautions, Notes:
© The Physics Classroom, 2009
The Laboratory
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The breaking strength of the string was measured in the Breaking Strength Lab during the
Newton's Laws unit. Students should use string from the same spool that was used during that lab.
The computer-interfaced force probe is the ideal tool for measuring the maximum load of the
string. Software accompanying the probes typically keeps track of the amount of force over the
course of time. The moment the string breaks, the tension reduces to zero. The force reported by
the computer just prior to this event is the maximum load.
Leaving the procedure open-ended makes this activity into an exceptional problem-solving
activity. Students are left to devise their own means by which to measure the maximum load.
A commonly used method involves pulling downward upon the string at its midpoint using the
computer interfaced force probe. Trials are repeated for the same length of string and the same
angle. An average value is determined from all the measurements.
Auxiliary Materials:
None
Scoring Rubric:
VF4.
Maximum Load Lab
Included, labeled and organized all parts of the lab report.
Data section includes an informative diagram of the experimental setup;
method of measurement is indicated. Data for several trials are organized;
unit is stated; outliers are indicated and an average maximum load is
calculated. Theoretical analysis based on the breaking strength is presented in
an organized fashion, beginning with a force triangle.
Conclusion/Discussion answers the question posed in the Purpose. An error
analysis is included; the reliability of the results are evaluated; a percent
difference calculation is performed and discussed; work is shown.
Score
_____/_____
Connections to The Physics Classroom Tutorial:
The following readings are a suitable accompaniment to this lab:
http://www.physicsclassroom.com/Class/vectors/u3l3b.cfm
http://www.physicsclassroom.com/Class/vectors/u3l3c.cfm
Connections to Minds on Physics Internet Modules:
Sublevels 3 and 4 of the Forces in Two Dimensions module are a suitable accompaniment to this lab:
http://www.physicsclassroom.com/mop/module.cfm
© The Physics Classroom, 2009